In spent lead acid battery recovery processes it is known to generally eliminate the sulphuric acid, a constituent of the electrolyte, and the sulphur, a constituent of the lead sulphate contained in the active mass, by treatment with alkaline or alkaline earth carbonate or bases. If for example sodium carbonate is used, sodium sulphate is formed during the treatment. The Na.sub.2 SO.sub.4 solutions are generally fed to the heavy metal removal plant and then discharged.
Only in certain more modern plants are the Na.sub.2 SO.sub.4 solutions fed after purification to a plant in which the pure anhydrous salt is crystallized, for sale. In this respect reference should be made for example to Italian patent No. 1,191,650 in the name of the present applicant.
However, in recent times, the market demand for pure sodium sulphate has decreased and the tendency is for this decrease to continue, with the result that processes which include the recovery of commercial anhydrous sodium sulphate have currently lost part of their initial interest.
A further drawback in this respect is that the soda required for desulphation of the active mass is often subject to market price rises, which makes its use inconvenient.
If the aforesaid recovery processes use calcium hydroxide for neutralizing the acid, the calcium sulphate is separated by filtration and the filter cake is despatched to controlled dumps. In this case there is an obvious serious environment impact as such residues always contain a certain amount of heavy metals.